One doesn't have to be hiking to come in contact with an itch-producing plant. Plants that transmit poison by contact can show up in backyards as well as in lakes and oceans. To protect yourself from a painful and irritating itch, it is a good idea to know what to look for before you venture outside.
Poison Ivy, Oak and Sumac
Probably the most well-known plants that cause itching are poison ivy, poison oak and poison sumac. They are all members of the same family and contain an oil called urushiol that causes an itchy skin rash in about 50 to 70 percent of the population. Poison ivy (Toxicodendron radicans) grows throughout the continental United States except the Southwest, usually along riverbanks. It has three shiny green leaves and a red stem. Poison sumac (Toxicodendron vernix) is a woody shrub that grows along the Mississippi River. Sumac has long stems with 7 to 13 oval leaves that are arranged in pairs on a red stem and topped with a single leaf. There are several different species of poison oak, all of which have three leaves that usually resemble that of an oak tree (hence the name). Species of poison oak are generally named by location: Western poison oak (Toxicodendron diversilobum) and Rocky Mountain poison oak (Toxicodendron rydbergii) grow in the western United States and Canada, Atlantic poison oak (Toxicodendron pubescens) and Eastern poison oak (Toxicodendron quercifolium) grow in the East.
Nettle is the common name for approximately 40 different species of plants of the genus Urtica. Most, but not all, have special needle-like hairs that break off and release a poison when touched. The poison can cause itching and pain. The most commonly found is stinging nettles (Urtica dioica and Urtica urens). Stinging nettles are tall and slender. The stems have four sides, and its leaves are egg-shaped with jagged edges.
The wood nettle (Laportea canadensis) is a cousin to the stinging nettle and can produce the same effect as a stinging nettle. However, the stinging hairs are found only on the stem and not on the egg-shaped leaves. The spurge nettle or bull nettle (Cnidoscolus stimulosus) is not a true nettle nor a relation to them. It gets its name because it is covered with stinging hairs. It has leaves with three to five lobes and produces a small white flower.
Itch-producing plants are not constricted to land, they also reside in both salt water and freshwater. Cyanobacteria, or blue-green algae, has been known to cause "swimmers itch" or "seaweed dermatitis." Several different species can cause itching, but the most common are Lyngbya majuscula, Schizothrix calcicola and Oscillatoria nigroviridis. All three of these live in tropical and subtropical waters. They are usually seen in the water when they are in bloom, and they will make the water appear like green paint or form a thick mat or foam on the shore.